We successfully did this the other day (although I didn't move us to window-aisle until the night before when it looked like there were a lot of empty seats -- five months out you'd have no idea).

They did clear a lot of standbys on the flight, but I think it still went out at least 40% empty. Presumably many of those standbys were nonrevs (at least one I noticed was an employee), but seems nobody wanted an EC middle over an aisle/window further in the back (if the GA even offered them).

On AA's 738s they physically block the four middle seats (with a tray table-like thing) in the first two rows behind the exit row (so they could get the pax count down to 150 and remove a FA). They have regular legroom, but guaranteed no middle passenger, sort of like European business. I generally prefer those to an extra legroom seat, because when you can angle your legs and spread out your stuff, it's a lot of extra room in general. Sadly, they're adding seats back with the US Air-ization of AA.